1. Field of the Invention
The present invention relates to a driving circuit for driving data lines of a liquid crystal display panel and a liquid crystal display apparatus using the same.
2. Description of the Related Art
As a man-machine interface, a flat panel display apparatus has been widely spread. Especially, a liquid crystal display apparatus is superior in manufacturing technique, yield and cost to other flat panel displays such as a plasma display apparatus. Thus, the liquid crystal display apparatus is applicable to various fields.
The liquid crystal display apparatus includes a display panel with a plurality of pixels arranged in a matrix. The display panel has two glass plates and liquid crystal material sealed in a gap between the glass plates. The liquid crystal material has the characteristic that the orientation of molecules is changed in accordance with an application voltage. The liquid crystal display apparatus uses its characteristic to display an image on the display panel. In short, the liquid crystal display apparatus controls the application voltage to each pixel and consequently changes a quantity of light transmitted through the two glass plates to display the image on the display panel.
As a driving method of displaying an image on a display panel, there are a simple matrix driving method and an active matrix driving method. At present, the liquid crystal display apparatus employing the active matrix driving method is generally used. An active element such as TFT (Thin Film Transistor) is provided for each pixel of the display panel in the active matrix liquid crystal display apparatus. Also, the display panel includes a plurality of scan lines and a plurality of data lines (signal lines) orthogonal to the plurality of scan lines. Also, each active element includes a gate electrode, a drain electrode and a source electrode. The gate electrode of each active element is connected to a corresponding one of the scan lines extending in a row direction. Similarly, the drain electrode of each active element is connected to a corresponding one of the data lines extending in a column direction. The active matrix liquid crystal display apparatus displays an image by using a displaying method typically called a sequential driving method. In the sequential driving method, the scan lines are sequentially scanned from an upper portion to a low portion or from the low portion to the upper portion on the display panel, and consequently displays an image on the display panel. This image is referred to as a frame (or a field).
When the display panel is driven, the continuous application of a DC voltage to the pixel cause deterioration of the liquid crystal material. The liquid crystal display apparatus typically employs a driving method called an inversion driving method, in order to prevent the deterioration of the liquid crystal material. In that method, the pixels in the liquid crystal display panel are driven in an AC manner while using the active matrix driving method. In the inversion driving method, the polarity of the pixel voltage to be applied is defined as a positive or negative voltage with respect to a voltage of a common electrode (a common voltage), and the polarity is inverted for every predetermined period. In short, in the inversion driving method, the voltage higher or lower than the common voltage is defined as a positive or negative voltage. Then, the positive voltage and the negative voltage are alternately applied to a pixel from the data line through the TFT for every predetermined period. Thus, the voltage to drive the data line is also inverted for every predetermined period.
As the inversion driving method used in the liquid crystal display apparatus, there are known the [Line Inversion Driving] method in which the polarity of the pixel voltage is changed for every data line in a row direction, and a [Dot Inversion Driving] method in which the polarity of the pixel voltage is changed for every pixel. The [Dot Inversion Driving] method is employed in the recent liquid crystal display apparatus of large scale and high definition. As the dot inversion drive method, there are known a 1-line dot inversion driving method in which the polarity of the pixel voltage is inverted each time one scan lines is scanned, and a 2-line dot inversion driving method in which the polarity of the pixel voltage is inverted each time two scan lines are scanned. With the 1-line dot inversion driving method and the 2-line dot inversion driving method, flicker and the like are reduced to improve the image quality.
With the larger scale and higher definition of the liquid crystal display apparatus, there is a case that a parasitic capacitance and parasitic resistance of the data line and the scan line are increased. The increase in the parasitic capacitance and parasitic resistance of the data line causes a waveform dullness of a driving voltage signal applied to the data line from a data line driving circuit. Thus, brightnesses are sometimes different between a pixel near to the data line driving circuit and a pixel distant from it. In order to solve such a problem, a technique is proposed in Japanese Laid Open Patent Publication (JP-A-Heisei 6-149183). In this conventional technique, data line driving circuits are provided on upper and lower sides of a panel, and are switched by setting two frames as one cycle. Thus, signal voltages are averaged, thereby reducing the deviation in the brightness.
In the dot inversion driving method, the display panel is driven in the positive and negative voltages with respect to the common voltage as the reference voltage. Thus, the display panel is driven by setting the two frames as one cycle.
FIG. 1 is a block diagram showing the configuration of a conventional liquid crystal display apparatus 101 employing the conventional dot inversion driving method. With reference to FIG. 1, the conventional liquid crystal display apparatus 101 is provided with a data line driving circuit (positive) 102a for supplying a positive signal, a data line driving circuit (negative) 102b for supplying a negative signal, a scan line driving circuit 103 for supplying a scan signal; a control circuit for outputting a clock signal and an image signal to be supplied to the data line driving circuit (positive) 102a and the data line driving circuit (negative) 102b; a display panel 105, a switch circuit 162 and a switch circuit 163. Also, the display panel 105 has data lines 107, scan lines 108 and a plurality of pixels 109. As mentioned above, the conventional display panel is driven by using a first frame and a second frame in one cycle.
FIG. 1 shows the liquid crystal display apparatus 101 of the first frame cycle. As shown in FIG. 1, the odd-numbered lines of the data lines in the liquid crystal display apparatus 101 are driven with the positive signal supplied from the data line driving circuit (positive) 102a in the first frame. The odd-numbered lines of the data lines are driven with the negative signal supplied from the data line driving circuit (negative) 102b in the second frame cycle. Here, it is supposed that the pixel near the data line driving circuit (positive) 102a is referred to as a pixel 109a, and the pixel distant from it is referred to as a pixel 109b. At this time, a difference between the common voltage and the pixel voltage applied to the pixel 109a is different from a difference between the common voltage and the pixel voltage applied to the pixel 109b. 
FIGS. 2A to 2D are timing charts showing the voltages applied to the pixels 109a and 109b. With reference to FIGS. 2A to 2D, the waveform of voltage signal on the data line in the first and second frames is shown by a solid line, and the waveform of the pixel voltage is shown by a dotted line. As mentioned above, the pixels 109a and 109b are connected to the odd-numbered data line 107 in the liquid crystal display apparatus 101.
The pixels 109a and 109b are driven by the positive pixel voltage in the first frame. The data line driving circuit (positive) 102a drives the pixels 109a and 109b with the positive voltage in the first frame. Since the pixel 109a is located close the data line driving circuit 102a, the voltage waveform of the pixel 109a on the data line 107 reaches a target voltage without any dullness. The voltage supplied from the data line 107 is applied to the liquid crystal through the TFT of the pixel. Since the ON resistance of the TFT is as high as several MΩ, the waveform of the pixel voltage is made dull, and the pixel voltage has the value of a positive voltage Va with respect to the common voltage. After that, the drive of a scan line associated with the pixel 109a is ended, and the pixel 109a holds the voltage Va. As shown in the timing charts of FIGS. 2A and 2B, the data line driving circuit 102b drives the pixel 109a with the negative voltage in the second frame. The pixel 109a is located distant from the data line driving circuit 102b. Thus, the voltage waveform of the data line 107 becomes dull. The drive of the scan line 108 associated with the pixel 109a is ended before reaching the target voltage. In response to the end of the scan line drive, the TFT is turned off. At this time, the pixel voltage has the value of a negative voltage Vb with respect to the common voltage, and the pixel holds the voltage Vb.
On the other hand, the data line driving circuit 102b drives the pixels 109b with the negative voltage in the second frame. The pixel 109b is located close the data line driving circuit 102b. Thus, the voltage waveform of the data line 107 reaches the targeted voltage without any dullness. For this reason, the pixel voltage has the value of a negative voltage Vc with respect to the common voltage. At this time, the waveform of the pixel voltage applied to the liquid crystal through the TFT of the pixel becomes dull due to of the ON resistance of the TFT. After that, the scan line drive is ended, and the pixel 109b holds the voltage Vc.
In the third frame, the pixel 109b is driven with the positive voltage. As shown in the timing charts of FIGS. 2A to 2D, the data line driving circuit 102a drives the pixel 109b with the positive voltage in the third frame. The pixel 109b is located distant from the data line driving circuit 102a. Thus, the voltage waveform of the data line 107 becomes dull, and the scan line drive is ended before reaching the targeted voltage. In response to the end of the scan line drive, the TFT of the pixel 109b is turned off. The pixel 109b holds a positive voltage Vd with respect to the common voltage.
Here, although the following voltage relationVa+Vb≈Vc+Vdis met, the brightnesses resulting from the positive voltage Va, the negative voltage Vb, the positive voltage Vd and the negative voltage Vc are slightly different from each other. This is because a positive gamma property and a negative gamma property are slightly different from each other.